Temperature compensation element for a traveling wave tube periodic array



y 21, 1964 HITOSHI KAJIHARA 3,142,003

TEMPERATURE COMPENSATION ELEMENT FOR A TRAVELING WAVE TUBE PERIODIC ARRAY Original Filed March 18, 1960 villi/116% Hai i/ II FIG. 3

a l/fr 2446/ ATTORNEYS United States Patent O" 3,142,093 TEMPERATURE COMPENSATION ELEMENT FOR A TRAVELING WAVE TUBE PERIODIC ARRAY Hitoshi Kajihara, Coytesville, N.J., assignor to General Precision, Inc, Little Falls, N.J., a corporation of Delaware (lriginal application Mar. 18, 1960, Ser. No. 16,104, now Patent No. 3,061,754, dated Oct. 3t 1962. Divided and this application May 1, 1962, Ser. No. 194,212 1 Claim. (Cl. 317-2tlll) The present invention relates to the magnetic structure associated with a traveling wave tube, and more particularly to the provision of temperature compensating means for use with such tubes.

A traveling wave tube, often referred to as a TWT, consists of a structure for producing an electron beam which traverses the tube; a transmission line often referred to as the slow wave structure which propagates a microwave signal in a manner permitting interaction between the electron beam and the signal; a collector for removing unused beam energy, transducers for introducing and removing this signal; and an attenuator which isolates the input and output sections of the slow wave structure to prevent oscillations. The structure for producing the electron beam is comprised of an electron source in the form of a cathode and one or more anodes or grids which control, guide and direct the electron beam. The slow wave structure may take the form of a helix or resonant cavities, or other means may be used to permit interaction between the beam and the signal.

The electron beam in the tube may be confined magnetically by employing a sinusoidally varying magnetic field. TWTs of this type, because of their field pattern are usually referred to as periodically focused traveling wave tubes. The magnetic structure itself is usually called a periodic TWT array or stack.

The introduction of periodic field focusing of low voltage electron beams was made by J. R. Pierce, Spacially Alternating Magnetic Fields for Focusing Low Voltage Electron Beams, Journal of Applied Physics, volume 24, page 1247, 1953. Later Mendel, Quate, and Yocom published the results of their work Electron Beam Focusing With Periodic Permanent Magnet Fields Proceedings IRE, volume 42, page 800, 1954. Still later, the design of periodically focused TWT arrays or stacks was described by Kern, K. N. Chang, in an article entitled Optimum Design of Periodic Magnetic Structures for Electron Beam Focusing, R.C.A. Review, volume 16, page 65, 1955. These articles as well as later patents and publications explain the disposition of the magnets as well as the means required to design the periodically varying magnetic structure. The described structures include a plurality of ring shaped magnets and pole pieces so disposed that adjacent poles of magnets are of the same polarity, i.e., North-South; adjacent South-North; adjacent North-South; etc. The axial field of the magnetic structure is made to coincide with the axis of the slow wave structure, i.e., the helix of the TWT. However, ferrite permanent magnets of the type used in periodic magnetic focusing arrays for TWTs exhibit a change in remanent induction with temperature. This property manifests itself in an array whose strength is temperature dependent, and, a TWT amplifier used with such an array or stack has a gain and power output which changes with temperature. In some instances the array strength may change sufficiently to reduce output power to zero. To obtain a TWT amplifier operating at peak performance over a range of temperatures, is therefore not possible with the stacks heretofore in use.

Although attempts were made to overcome the foregoing difficulties, so as to provide a stack not subject to 3,1423% Patented July 21, 1964 ice changing magnetic characteristics because of temperature variations, none, as far as I am aware, was entirely successful when carried into practice commercially on an industrial scale.

Thus, it is an object of the present invention to provide a magnetic stack whose magnetic characteristics will not be affected by temperature changes between a range of about 65 C. to about C.

Another object of the present invention is to provide such a stack without increasing the size and Weight of the stack to any appreciable extent.

Still another object of the present invention is to provide such a stack without changing the geometry of the stack to any great extent.

With the foregoing and other objects in View, the invention resides in the novel arrangement and combination of parts and in the details of construction hereinafter described and claimed, it being understood that changes in the precise embodiment of the invention herein disclosed may be made within the scope of what is claimed without departing from the spirit of the invention.

The accompanying drawings, illustrative of one embodiment of the invention, and several modifications thereof, together with the description of their construction and the method of operation and utilization thereof, will serve to clarify further objects and advantages of my invention.

Other advantages will become apparent from the following descripton taken in conjunction with the accompanying drawing in which:

FIGURE 1 is a longitudinal cross-sectional view of a portion of a periodic array of the prior art showing the main flux paths of the array;

FIGURE 2 is a view similar to FIGURE 1 showing one type of temperature compensation element used with the periodic array; and

FIG. 3 is a perspective side view of a compensating element of the type depicted in FIG. 2.

In the drawing, there is first illustrated the principal fields of magnetic fiux for an optimized temperature uncompensated array 11. For the purpose of this invention, an optimized array is one which yields a specified field strength of a given periodicity and array interior diameter with a minimum of array outer diameter. Attention is directed to the fact that for this array, the magnets 12 extend somewhat beyond the pole pieces 13 and the magnet interior diameter 14 is kept at a minimum. The array illustrated in FIG. 1, made of ferrite magnets exhibits a decrease in array strength with increasing temperature. Consequently, the lowest field occurs at the highest operating temperature.

The main flux paths of the periodic array structure are from one pole down to the cylindrical axis centerline and over to the other pole, which is termed herein as path I; across the inner rim of the ring shaped magnets from pole to pole parallel to the cylindrical axis which is termed herein as path II; and from pole to pole across the outer rim of the ring shaped pole pieces which is termed herein as path III. Since the electron beam in the TWT is acted on principally by path I, the objective of temperature compensation is to maintain the flux of path I constant. This can be achieved by varying the permanence of paths II and III appropriately with changing temperature. The permanence variation is obtained by inserting material whose permeability changes appropriately with temperature in paths II and III. An ideal compensator material is characterized by high saturation flux density and high permeability, the latter decreasing linearly with temperature. Its Curie temperature coincides with the highest array operating temperature.

For the purpose of the present invention, it has been Composition of Temperature Compensator Alloys Carbon 30. 89 balance balance balance Generally speaking, the present invention contemplates providing an array or stack having a constant field strength over a wide temperature range, e.g., from about -65 C. to about +125 C. by having an array design of minimum size with the required field strength at the highest operating temperature and, in combination therewith compensator elements disposed across paths II and III or III to maintain constancy of field around the stack cylindrical axis with decreasing temperature.

According to the invention, there is provided a ring-like compensator 14 surrounding the cylindrical axis of the array. For convenience, this type of compensator is termed a type A compensator. Type A compensators permit the maintenance of the optimized pole piece to magnet dimensional relationship. It diverts the maximum amount of flux per unit cross sectional area. The magnitude of flux diverted from path I determines the cross sectional area required. However, the area which can be introduced is fixed by the magnets and pole piece interior diameters. Thus, type A compensator preferably takes the form of a disk-shaped ring, i.e., there is a ring portion 14a and a flange portion 14b. The flange shaped portion is so disposed as to fit on the hub of the pole piece.

It is to be observed therefore that the present invention provides for an article of manufacture, namely a stack having a magnetic field around the inner cylindrical axis thereof of a fairly uniform field strength over a wide temperature range comprising in combination, a plurality of ring-shaped magnets of the same size and magnetic characteristics, cylindrically aligned; disk-shaped pole pieces interposed between each of said magnets and having a ring-like aperture substantially the size of the magnet ring aperture; and compensators made of a nickel-iron alloy having between about 28.5% to about 33.5% nickel and the balance substantially iron interposed across one of the principal magnetic paths of the stack, i.e., across the inner rim of the ring shaped magnets parallel to the cylindrical axis of the stack. Said compensators are ring shaped and have a flange, i.e., adapted to go on the hub of the pole piece in the stack as well as a disk adapted to lie between pole pieces just surrounding the cylindrical axial aperture of the stack.

It is likewise to be observed that as used herein, the term stack has a special meaning and refers to the periodic array of a traveling wave tube, i.e., a plurality of aligned cylindrical magnets, pole pieces, etc. used to vary the TWT beam sinusoidally, and the term element is not used in the chemical sense but in the mechanical sense and as such has a meaning somewhat similar to the term member.

This application is a division of the Hitoshi Kajihara US. patent application Ser. No. 16,104 filed March 18, 1960, now US. Patent No. 3,061,754.

Although the present invention has been described in conjunction with preferred embodiments, it is to be understood that modifications and variations may be resorted to without departing from the spirit and scope of the invention, as those skilled in the art Will readily understand. Such modifications and variations are considered to be within the purview and scope of the invention and appended claim.

I claim:

A stack having a magnetic field around the inner cylindrical axis thereof of a fairly uniform field strength over a wide temperature range comprising in combination, a plurality of ring-shaped magnets of the same size and magnetic characteristics, cylindrically aligned; disk-shaped pole pieces interposed between each of said magnets having a ring-like apertured hub substantially the size of the magnet ring aperture; and, compensator-s made of a nickel-iron alloy having between 28.5% to about 33.5% nickel and the balance substantially iron, said compensators each being a disk-shaped ring comprising a flange portion disposed over the inner hub of the pole piece adapted to go below the pole piece in the stack and a ring portion adapted to surround the cylindrical axial aperture of the stack and lie between pole pieces.

References fitted in the file of this patent UNITED STATES PATENTS 

